Alpha-lipoic acid was first isolated as an acetate replacing factor. It is slightly soluble in water and soluble in certain organic solvents. Alpha-lipoic acid was initially identified as a vitamin after its isolation, but it was later found to be synthesized by mammals, including humans, as well as by plants. The complete enzyme pathway that is responsible for the de novo synthesis has not yet been definitively elucidated. Several studies have indicated that octanoate serves as the immediate precursor for the 8-carbon fatty acid chain, and cysteine appears to be the source of sulfur. As an amide (lipoamide), it functions as a cofactor in the multienzyme complexes that catalyze the oxidative decarboxylation of alpha-keto acids such as pyruvate, alpha-keto glutarate, and branched chain alpha-keto acids.
Alpha-lipoic acid is one of the strongest naturally occurring antioxidants. Alpha-lipoic acid (LA) is also known as thioctic acid, 1,2-dithiolane-3-pentanoic acid, 1,2-dithiolane-3-valeric acid and 6,8-thioctic acid. Alpha-lipoic acid has a chiral carbon atom and occurs in two enantiomeric forms (R— and S—). The form of alpha-lipoic acid sold in stores is a synthetic mixture of the natural isomer (R—) and the unnatural isomer (S—). The natural form of R-LA is not as stable as the synthetic mixture. One manufacturer, Asta Medica, sells R-LA for diabetes and has made a stable form of R-LA by crystallizing it with Tris buffer, a commonly used synthetic, but unnatural, buffer.
Various enantiomeric forms of alpha-LA, and combinations and derivatives thereof (including its reduced form), have been used to treat numerous conditions. For example, LA's have been used in the treatment of circulatory disorders. LAs and vitamins have been found useful for producing analgesic, anti-inflammatory, antinecrotic, anti-diabetic and other therapeutic effects. Certain alkylated derivatives of LA have been used in treatment of retroviral diseases.
Alpha-lipoic acid, and its reduced form, dihydrolipoic acid (DHLA) have antioxidant properties. Lipoate (a term for carboxylic acid esters and salts), or its reduced form, DHLA, reacts with reactive oxygen species such as superoxide radicals, hydroxyl radicals, hypochlorous acid, peroxyl radicals, and singlet oxygen. It also protects membranes by interacting with vitamin C and glutathione, which may in turn recycle vitamin E. In addition to its antioxidant activities, DHLA may exert prooxidant actions to reduction of iron. Alpha-lipoic acid administration has been shown to be beneficial in a number of oxidative stress models such as ischemia-reperfusion injury (IRI), diabetes (both alpha-lipoic acid and DHLA exhibit hydrophobic binding to proteins such as albumin, which can prevent glycation reactions), cataract formation, HIV activation, neurodegeneration, and radiation injury. Furthermore, lipoate can function as a redox regulator of proteins such as myoglobin, prolactin, thioredoxin, and NF-kappa-B transcription factor.
Lipoate may also have other activities. For example, DHLA has been found in vitro to be an anti-inflammatory agent which at the same time interferes with nitric oxide release from inflammatory macrophages and protects target cells from oxygen radical attack.
Lipoic acid is also a coenzyme for several enzymes. Lipoic acid is a coenzyme for both alpha-keto acid dehydrogenase complex enzymes (i.e. pyruvate dehydrogenase complex and alpha-keto glutarate dehydrogenase complex), branched chain alpha-keto acid dehydrogenase complex, and the glycine cleavage system. In the enzyme system, the body forms a multi-enzyme complex involving lipoic acid, that breaks down molecules of pyruvate produced in earlier metabolism, to form slightly smaller, high energy molecules, called acetyl-coenzyme A. This results in molecules that can enter into a series of reactions called the citric acid cycle, or Krebs cycle, which finishes the conversion of food into energy. Essentially, lipoic acid stimulates basal glucose transport and has a positive effect on insulin stimulated glucose uptake.
Under physiological conditions, LA exists as lipoamide in at least five proteins where it is covalently linked to a lysyl residue. Four of these proteins are alpha-ketoacid dehydrogenase complexes, the pyruvate dehydrogenase complex, the branched chain keto-acid dehydrogenase complex and the alpha-ketoglutarate dehydrogenase complex. Three lipoamide-containing proteins are present in the E2 enzyme dihydrolipoyl acyltransferase, which is different in each of the complexes and specific for the substrate of the complex. One lipoyl residue is found in protein X, which is the same in each complex. The fifth lipoamide residue is present in the glycine cleavage system.
Recently LA has been detected in the form of lipoyllysine in various natural sources. In the plant material studied, lipoyllysine content was highest in spinach. When expressed as weight per dry weight of lyophilized vegetables, the abundance of naturally existing lipoate in spinach was over three- and five-fold higher than that in broccoli and tomatoes, respectively. Lower concentrations of lipoyllysine were also detected in garden pea, Brussels sprouts and rice bran.
In animal tissues, the abundance of lipoyllysine in bovine acetone powders can be represented in the following order of 1) kidney, 2) heart, 3) liver, 4) spleen, 5) brain, 6) pancreas and 7) lung.
LA suffers from certain disadvantages, however. In particular, the natural form R-LA is unstable above 40° C., so it can degrade under some warehousing conditions. Also LA is hygroscopic. What is needed is stabilization of this natural form of LA with a natural salt.
Therefore, a need exists for compositions and/or methods to prepare alpha lipoic acid compositions in a manner that overcomes one or more of the identified current drawbacks of available materials.